The human intestine harbors nearly 100 trillion bacteria that are essential for health. These organisms make critical contributions to human metabolism by helping to break down complex polysaccharides that are ingested as part of the diet. However, certain members of this microbial community can invade the intestinal barrier and cause opportunistic infections that can lead to bacteremia and sepsis. Further, they can trigger detrimental inflammatory responses, leading to inflammatory bowel disease. However, little is known about what factors predispose normal intestinal bacteria to transition from benign symbionts to invasive pathogens. Enterococcus faecalis is a predominant member of the mammalian intestinal microbiota that can opportunistically disseminate from the intestine and cause disease. It is one of the most common causes of hospital-acquired bloodstream infections and is a major cause of endocarditis. The research outlined in this proposal will uncover the molecular factors that promote the transition from a symbiotic to a pathogenic lifestyle in E. faecalis. Based on preliminary data, one of the key factors governing this transition is environmental cues that are encountered by the organism when it attaches to the surface of the intestinal tract. This proposal will 1) Determine which environmental cues alter E. faecalis gene expression during association with the intestinal surface; 2) Identify E. faecalis genes that are important for attachment to and invasion of the intestinal barrier; and 3) Determine how the host immune response minimizes E. faecalis attachment to and invasion of the intestinal barrier. These studies should shed light on the mechanisms used by bacteria to transition from a commensal lifestyle to a pathogenic state in the host. Furthermore, these studies will likely identify candidate targets for the development of novel IBD therapeutics and treatments for opportunistic infections.